THE GEOLOGY OF THE LEA VALLEY. 



199 



If we note the formations traversed by the southern tributaries 

 of the Thames east of Windsor, the \\'ey, Mole, Darent, and Med- 

 way, we find that they all rise in the Lower Greensand or Wealden 

 districts, beyond the Chalk of the North Downs. Only the smaller 

 streams, such as the Cray, Wandle, and Ravensbourne, have their 

 sources within the Chalk and Tertiary area. 



But the northern tributaries of the Thames in the district 

 between Windsor and the sea are all Chalk and Tertiary streams. 

 The chief of these are the Colne and Lea (with the smaller rivers 

 which fall into them), and the Lea is decidedly the more important 

 of the two. It " rises from the lowest part of the Chalk, north-east 

 of Dunstable " (Whitaker : " Geology London Basin," part i., p. 3), 

 very close to the Chalk boundary. Its chief tributaries, the 

 Mimram, Beane, Rib, Quin, and Stort, also originate in the Chalk, 

 which, in the uppermost part of their courses, is either bare or 

 covered by Glacial Drift, which consists of sand and gravel overlain 

 by Boulder Clay. The sand and gravel appear in the flanks of the 

 valleys which have been cut by the various streams, while the 

 Boulder Clay forms the surface of the higher ground between them. 

 Along a somewhat irregular line, which may be roughly described as 

 ranging from Sudbury to Bishop Stortford, and thence towards 

 Hatfield and Watford, Woolwich and Reading Beds have been 

 detected here and there beneath the Glacial Drift. But the belt of 

 ground occupied by them is a very narrow one, while the overlying 

 London Clay may be seen either forming the surface, or more or 

 less covered by superficial beds, over four-fifths of Essex. It forms, 

 with the occasional addition of a capping of some superficial bed 

 of gravel or Boulder Clay, all the higher ground on each side of the 

 Lea valley below the junction with the Stort, beyond the alluvium of 

 the marshes, and the older and slightly more elevated flats of gravel 

 and brick-earth close to the river. These last-named deposits are 

 entirely due to the action of the stream, and have been formed in 

 the following manner. 



In addition to the action of rivers in cutting their beds deeper 

 and deeper, and further and further back, they also tend to change 

 their courses laterally, to eat into the bank on the side where the 

 current is strong, and to deposit material on the other where it is 

 sluggish. This, in the case of the Lea, has resulted in the destruction 

 of much high ground, consisting chiefly of London Clay, and the 

 deposition, on the planed-down surface, of a comparatively thin bed 



